Found a bug of the "I have no idea how that could have possibly worked" kind:

class arena {
  std::vector<std::unique_ptr<intptr_t[]>&gt; allocs_;
public:
  intptr_t* allocate(size_t n) {
    allocs_.emplace_back(new intptr_t[n]);
    return allocs_.end()-&gt;get();
  }
};

</std::unique_ptr<intptr_t[]>

Notice that instead of returning the last element, the code returns the end() iterator which points to after the last element. To random memory. Oops?..